The present invention relates to nuclear reactor plants and, more particularly, to a plant depressurization system. A major objective of the present invention is to provide for simple and reliable handling of steam generated during a plant shutdown.
In a nuclear reactor, heat is generated by fissioning within a reactor core. In a boiling-water reactor (BWR), this heat is used to boil water circulating within a reactor pressure vessel enclosing the core. The steam so generated can be used to drive a turbine, which in turn can drive a generator to produce electricity.
Under certain exigencies, the turbine can become decoupled from the reactor. In response to this decoupling, control rods are automatically fully inserted to minimize further fissioning and, thus, heat generation. Heat already generated in the core continues to boil water, which no longer is condensed by the transfer of energy to the turbine. Pressure due to the accumulating steam is relieved by releasing steam from the reactor pressure vessel via a piped connection to the wetwell in the containment where the released steam is condensed.
After the control rods are fully inserted, fissioning falls to about 1% of reactor power within three hours and to about 0.5% within one day. To prevent core overheating, coolant released to the containment is replenished by water delivered from a gravity-driven cooling system (GDCS).
It is economically undesirable to build a wetwell structure large enough to handle all the steam that can be generated by decay heat over a period of days. Instead, a reactor plant is designed so that, so far as is possible, the heat is removed from the containment, relieving the pressure buildup. In existing reactor plant designs, this is done by cooling the wetwell using heat exchangers and pumps. In newer passive designs the use of pumps is not desired and excessive pressure buildup and consequent failure of the containment must be prevented even in the absence of electrical power to drive the pumps.
One option is to condense the steam on steel containment walls. This process can be facilitated by spilling water on the exterior of the containment walls. However, in addition to condensable steam, the wetwell contains noncondensable gases, such as nitrogen, that fill the drywell before the isolation event. The noncondensable gases can accumulate near containment walls, inhibiting heat transfer from steam to the walls, thus limiting condensation. Moreover, the surface area required for adequate pressure relief results in a huge containment structure. A huge containment structure is more costly and is more subject to defects than a more compact structure. Thus, an approach to pressure relief is required that permits a more compact structure.
A more economical approach is to duct released steam from the reactor vessel to an isolation condenser. A typical isolation condenser is a heat exchanger including a multitude of small-diameter tubes submerged in a pool of water. As steam flows through these tubes, heat is transferred from the steam through the tube walls to the condenser pool. Once the condenser pool boils, the steam so generated can be vented to the environment since it contains no radioactive material. Isolation condensers are also subject to the insulating effects of noncondensable gases, but proper design can lead to the evacuation of the noncondensable gases from the tubes, facilitating heat transfer and steam condensation.
A concern with the use of isolation condensers is that the relatively thin condenser tube walls become, in effect, a weak link in the primary system boundary, as the condenser is normally connected to the reactor vessel and is at full reactor pressure. Failure of the thin condenser walls will lead not only to a slow depressurization of the reactor vessel but also forms a direct pathway for radioactive products in the vessel to the environment, thereby bypassing the containment.
In addition, an isolation condenser presents a complex structure that must be periodically maintained. Preferably, the isolation condensers are made compact to permit their removal for repair. This poses serious problems on the flow stability of the exterior side of the condenser. The isolation condenser will thus be designed to have a minimum surface area, in order to reduce the probability for leakage. This is achieved in limiting the design to handle the heat load only at full pressure (7 MPa) of the vessel and with no noncondensables present. The conditions are quite different after the failure of systems or lines connected to the reactor vessel or of the reactor vessel itself. Steam and/or hot water will then be released into the drywell in the containment that surrounds the reactor vessel and the attached systems. The volume of the wetwell is however not sufficient to handle all the steam that can be generated by decay heat over a period of days. Accordingly, an effective and economical steam pressure relief system is required which addresses the insulating effects of noncondensable gases, maintains a secure containment boundary, and permits convenient inspection and repair.